<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin: 0px;
            padding: 0px;
        }
        #menu{
            width: 300px;
            height: 50px;
            background-color: red;
            text-align: center;

        }
        #menu:hover #selectitem{
            top: 1px;
            
        }
        span{
            color: #fafafa;
            font-size: 20px;
            line-height: 50px;
        }
        #selectitem{
            width: 300px;
            height: 300px;
            background-color: #efa0a0;
            position: relative;
            top: -300px;
            z-index: -1;
            transition: 1s;
        }
        .item{
            display: flex;
            height: 600px;
        }
        .item:hover .son-itemi{
           position: relative;
           left: 0px;
        }
        .itemtext{
            width: 300px;
            background-color: aqua;
            
        }
        .son-itemi{
            width: 300px;
            background-color: aqua;
            position: relative;
            left: -300px;
            z-index: -1;
        }

    </style>
</head>
<body>
    <div id="menu">
        <span>菜单选项</span>
        <div id="selectitem">
            <div class="item">
<div class="itemtext">
    <span>选项</span>
</div>
    <div class="son-itemi">
<div class="son-item">
       <span>子选项</span>
</div>
<div>
    <span>子选项</span>
</div>
<div>
    <span>子选项</span>
</div>
  </div>
    </div>
      
<div class="xiak"><span>选项</span></div>
<div class="xiak"><span>选项</span></div>
<div class="xiak"><span>选项</span></div>
<div class="xiak"><span>选项</span></div>
<div class="xiak"><span>选项</span></div>
        </div>
    </div>
    
</body>
</html>